It is often desirable during medical diagnostic procedures to monitor the patient's blood on a continuous basis for such characteristics as the concentrations of oxygen (O.sub.2) and carbon dioxide (CO.sub.2), and the pH level. Considerable research and development efforts have been directed to in vivo sensing devices that can be delivered to and placed directly in the patient's artery to expose the sensor to blood flowing in that artery. Such continuous in vivo sensing provides a real time monitoring of the patient's condition and is preferred, in many cases, to a batch processing in which a predetermined sample volume of blood is removed and tested outside the bloodstream.
A variety of blood gas sensing devices have been proposed, typically in the form of an optical probe that incorporates a light transmissive optical fiber having a sensing element at the distal end of the fiber. By way of example, a number of such sensing devices are described in "Progress in the Development of a Fluorescent Intravascular Blood Gas System In Man", C. K. Mahutte et al., Journal of Clinical Monitoring, Vol. 6, No. 2, April 1990, pp. 147-157, and J. L. Gehrich et al., "Optical Fluorescence and Its Application to an Intravascular Blood Gas System", IEEE Trans. Biomed. Eng., vol. 2, 1986, pp. 117-132, the disclosures of which are hereby incorporated by reference in their entirety. Reference also is made to U.S. Pat. No. 4,830,013 (issued May 16, 1990) and U.S. Pat. No. 4,951,669 (issued Aug. 28, 1990), both to Maxwell et al., for further descriptions of such probes.
Typically, it has been proposed to insert a blood gas sensor into an artery, such as the radial artery, in the region of the wrist. Access to the radial artery is convenient in that it is common to catheterize hospital patients with a short catheter accessing the radial artery for measuring blood pressure. It would be desirable to insert a blood gas sensor probe through that same catheter, thus avoiding the necessity of making a second percutaneous puncture.
Although considerable progress is believed to have been made in the development of the technology of the sensors themselves, significant difficulties have been encountered in using such sensors to measure the blood parameters under investigation. Although it would be desirable to be able to measure the blood parameters directly and continuously within the artery, attempts to do so frequently have resulted in aberrant, erratic sensor values. A number of hypotheses have been developed to explain the cause of the aberrant sensor values, among which is one referred to in the Mahutte publication as the "wall effect", said to be caused by the sensor touching the blood vessel wall and hence resulting in a reading of an average of blood and tissue rather than of the blood alone. Also suggested as a possible cause for the aberrant sensor values has been clotting at the probe tip. It was stated in Mahutte that when the probe tip was retracted within the delivery catheter and no longer touched the arterial wall, certain aberrations in sensed values were allegedly eliminated. However, retraction of the probe within the cannula tip causes many other difficulties. In an attempt to measure oxygen with the probe tip retracted in the cannula, it was necessary to withdraw a sample of the blood into the cannula to contact the tip of the retracted sensor. Thus, the sensing is not continuous but, instead, is periodic as a "batch" of blood is drawn into the cannula into contact with the sensor. Further, the blood inside the cannula may not have the same parameters as the blood flowing in the artery, especially when it becomes mixed with saline or other fluids which pass through the cannula. The result is an inaccurate measurement and a long response time. Additionally, retraction of the sensing probe within the catheter tip interferes with fluid communication through the catheter and has a damping effect on blood pressure readings. Still further it may cause clotting. Thus, the proposed solution in Mahutte, of retracting the probe within the catheter, has not been found acceptable.
It would be desirable, therefore, to provide an improved system by which a probe sensor may be placed within the artery so as to be presented to a continuous flow of blood, but in a manner in which the probe sensor is spaced substantially from the inner surface of the blood vessel wall, in order to accurately measure a parameter of the blood. It is among the objects of the invention to provide such a system.
Another object is to provide such a system which allows a probe to be positioned through a catheter without substantially effecting the taking of blood pressure measurements.
Yet another object is to provide such a system which prevents thrombus formation.
A still further object is to provide a system for positioning any type of probe spaced from a body member.